Induction motor problem (help wanted)

[jonny boy]

What adverse effect can setting the amperage dial in the starter have on the motor then?

jon.

 

[9fingers]

Surely the overload relay reacts to temperature thta's why they are known as 'thermal overload relays'? The bi-metal strips inside heat up as the current increases and bend due to the temperature increase causing the trip mechanism to operate.

NO, Thermal overload relays use a thermal method to integrate the current. The trip mechanism has absolutely nothing what so ever to do with the temperature of the motor.
How can it when they are separated by a length of cable?

Bob

 

[herdsman]

There is a certain tolerance to allow for a slightly higher setting as already stated in this thread.

Are he drive belts too tight? What is the voltage at the motor starter before switching on?

 

[9fingers]

Bob, I have tried it when the starter is set on 12 just over. It say's inside the starter cover, do not exeed the FL amperage of the motor or motor burnout could occur. I dont want to risk that.

jon.

I think you said you are running a 60hz motor on UK 50hz supply. This will push up the motor current by 20% so I maintain you will need to ush up the current setting on the trip.

The label could well read " running a 60Hz motor on 50Hz can increase the risk of burning out the motor" but you are prepared to take that "risk"

Bob

 

[jonny boy]

Fair comment Bob, I have an extractor that only has a two horse motor and in that starter, the dial is set at 17 amps, that was set by Axminster and has never tripped in two years. Are you fairly certain I could raise the dial in the saw's starter then?

jon.

 

[herdsman]

Surely the overload relay reacts to temperature thta's why they are known as 'thermal overload relays'? The bi-metal strips inside heat up as the current increases and bend due to the temperature increase causing the trip mechanism to operate.

NO, Thermal overload relays use a thermal method to integrate the current. The trip mechanism has absolutely nothing what so ever to do with the temperature of the motor.
How can it when they are separated by a length of cable?

Bob

The voltage supplying the motor must first pass through the themal relay otherwise the motor will not be protected, I agree that the temperature of the motor does not operate the trip but the currect that is drwan by the motor will also pass through the thermal relay. The cables from the supply to the motor are part of the same cirucuit and therefore it is not separate.

 

[herdsman]

Fair comment Bob, I have an extractor that only has a two horse motor and in that starter, the dial is set at 17 amps, that was set by Axminster and has never tripped in two years. Are you fairly certain I could raise the dial in the saw's starter then?

jon.

FLC for a 1.5 kW motor is 17.4 amps

 

[jonny boy]

If the worst comes to the worst, i'll get a new motor for it but are we basically saying that anyone who has an induction motor run machine from the US will have experienced the same?

jon.

 

[jonny boy]

Herdsman, How is it then, that on the saw plate, it say's the motor is rated at 12.4 amps. Or is this NOT the full load amperage?

jon.

 

[9fingers]

Fair comment Bob, I have an extractor that only has a two horse motor and in that starter, the dial is set at 17 amps, that was set by Axminster and has never tripped in two years. Are you fairly certain I could raise the dial in the saw's starter then?

jon.

Yes I would set it a bit higher in 1 amp steps and record the time to cut out at each step.
When the time taken to trip is getting long enough then stop raising the set point.

What is the rated current on the axminster motor plate compare to the 17 amp setting? you could use this as a guide/comfort factor for setting the TS trip.

I am in a separate email exchange with someone else who is running an imported TS on UK mains to find out the ratio of plated current to trip current. When I have a result I will mail you.

Bob

 

[9fingers]

Surely the overload relay reacts to temperature thta's why they are known as 'thermal overload relays'? The bi-metal strips inside heat up as the current increases and bend due to the temperature increase causing the trip mechanism to operate.

NO, Thermal overload relays use a thermal method to integrate the current. The trip mechanism has absolutely nothing what so ever to do with the temperature of the motor.
How can it when they are separated by a length of cable?

Bob

The voltage supplying the motor must first pass through the themal relay otherwise the motor will not be protected, I agree that the temperature of the motor does not operate the trip but the currect that is drwan by the motor will also pass through the thermal relay. The cables from the supply to the motor are part of the same cirucuit and therefore it is not separate.

Herdsman: please note that I said the motor and thermal relays were separated BY a cable not that the cable was separate.
Voltages do not pass through relays - currents pass through things and current is what the overload relay measures.

Bob

 

[jonny boy]

Good idea about raising it 1amp at a time to see if the time to trip gets longer. The motor plate on the extractor doesn't have rated amp on it. I've already had a look to compare but that motor is a 1.5kw output (two horse) whereas the saw is a three horse, so I would have thought that if the extractor is set at 17 amp and the saw at 12, this is not high enough?
Just as a thought, could the dial be set at 12 because it has origionally been used in the US on a 60hz supply?

jon.

 

[herdsman]

Surely the overload relay reacts to temperature thta's why they are known as 'thermal overload relays'? The bi-metal strips inside heat up as the current increases and bend due to the temperature increase causing the trip mechanism to operate.

NO, Thermal overload relays use a thermal method to integrate the current. The trip mechanism has absolutely nothing what so ever to do with the temperature of the motor.
How can it when they are separated by a length of cable?

Bob

The voltage supplying the motor must first pass through the themal relay otherwise the motor will not be protected, I agree that the temperature of the motor does not operate the trip but the currect that is drwan by the motor will also pass through the thermal relay. The cables from the supply to the motor are part of the same cirucuit and therefore it is not separate.

Herdsman: please note that I said the motor and thermal relays were separated BY a cable not that the cable was separate.
Voltages do not pass through relays - currents pass through things and current is what the overload relay measures.

Bob

My apologies for misreading your text, I agree that the thermal relay reacts to current but it is heat generated by current that causes the reaction, and surely voltage does pass through relay contacts in order to reach other parts of the circuit and if voltage does not pass through the relay then there will be no current because you need voltage and resistance to get a current?

 

[9fingers]

Herdsman, How is it then, that on the saw plate, it say's the motor is rated at 12.4 amps. Or is this NOT the full load amperage?

jon.

Jon, the current drawn at full load depends on the motor design and the frequency of operation.

Motors of the same nominal horsepower will have different fl currents.

I'll go down to my workshop now and have a look at some motor rating plates.

Bob

 

[herdsman]

It will be interesting to find out the solution to this one :shock:

Bob, you are earning your supper tonight!

 

[jonny boy]

I do appreciate all the members taking the time to follow this thread.

jon.

 

[herdsman]

I do appreciate all the members taking the time to follow this thread.

jon.

Jon,

We will all learn something from this and I know how frustrating it can be when you want to use a machine but it does not want to play.

 

[9fingers]

Some motors from around the workshop yield the following:-

1/2 HP 3.8amps
1.5 HP 7amps
2.0HP 6.9 amps (!!)
3hp 15amps

all 50Hz single phase motors

I can't read the table saw motor current as the universal joint in my neck refuses to play!! but it is 3hp and the thermal trip is set at 18 amps (as it was delivered to me as new)

Bob

 

[9fingers]

Bob, you are earning your supper tonight!

Well I try to help with electrical/mechanical problems in return for the huge amount I learn about wood by lurking here and the 'other place'

Tonight the replies were coming in faster than I could respond!!


Bob

 

[jonny boy]

Thanks for that Bob. I'm going to try it on about 15amps then if still cutting out in the same time span, i'll take the motor to Beatson fans and motors.(local motor rewinders+engineers)
More on this tommorrow.
cheers,
jon.